A friend of mine has a chinese electric scooter (9KW hub motor) whose rear wheel started to seize after a few Km, it looks like it seizes due to warm up. Letting it cool a little it will spin freely again. I made a few searches but couldn't find anything similar, maybe I'm not using the right words. Any input on this, what may be the cause? The wheel starts seizing progressively until it gets completely blocked. After some time (cooling... we supose) it unblocks again.

Sounds like it could be a short in the phase wiring. Might not short out until something heats up and expands. Inspect the wiring coming out of the axle first, most likely place to get a nick in the insulation.

also when the motor seizes check the resistance of the phase wires. you might have a dead short. then the motor will not move unless you put a prybar on it. with smaller motors you can overcome this by hand, but with a 9kW motor you have way too much magnetic holding power to move it by hand.